Calcium exists in vivo as a component of bones, and it also causes important actions on contraction of muscles, excitation transmission of nerves, activation of enzymes, secretion of hormones, and the like. Specifically, calcium plays a role in the regulation of cell osmotic pressure, antagonism to Na or K ions, regulation of excitability of muscles and nerves, stimulus transmission of nerves, and blood coagulation, and as an activation factor for enzyme activity. Therefore, the real-time measurement of the distribution and changes of in vivo calcium ions is important in the discovery of diseases, therapeutic courses, research of physiological functions, and the like. The extracellular calcium ion concentration is 1 to 2 mM, while the intracellular calcium ion concentration is 50 to 100 mM, which is one ten-thousandth of the extracellular one.
As a sensor of such calcium ions, there has been reported, for example, a compound having a glycol ether diamine tetraacetic acid (ethylene glycol tetraacetic acid: EGTA) skeleton (Fura-2) (Non Patent Literature 1). However, since this sensor has a high association constant pKa of about 6 to 10, the sensor can detect a calcium ion concentration of the order of nM but cannot detect the extracellular calcium ion concentration. Further, the sensor only has an irreversible sensing capability at the extracellular calcium concentration, and is difficult to be integrated with a device, and therefore the sensor is only used in in vitro intracellular imaging.